1. Field of the Invention
The present invention relates to an apparatus and method for connecting a call in a wireless communication system, and more particularly to an apparatus and method for connecting a radio link in a wireless communication system.
2. Description of the Related Art
A cellular mobile communication system may be mentioned as a typical example of a wireless communication system. This cellular mobile communication system has been developed to provide users with voice communications wherever they are located. However, in line with the rapid growth of wireless communication technology, the wireless communication system has evolved in the form of a system that can provide mass data at high speed according to demands of users who want to be provided with mass data at high speed.
Such a wireless communication system may be broadly divided into the North American mobile communication system employing a synchronous scheme and the European mobile communication system employing an asynchronous scheme. The UMTS (Universal Mobile Telecommunication Service) system belonging to the European mobile communication system is a 3rd generation asynchronous mobile communication system which is based on GSM (Global System for Mobile Communications) and GPRS (General Packet Radio Services) and employs a WCDMA (Wideband Code Division Multiple Access) scheme. The 3GPP (3rd Generation Partnership Project) responsible for UMTS standardization is currently discussing LTE (Long Term Evolution) as a next generation mobile communication of the UMTS. LTE with an aim of commercialization by around 2010 is technology for implementing high-speed packet-based communication with a transfer rate of max. 100 Mbps.
In order to satisfy the above conditions, various plans for applying to the LTE system are under discussion, including a plan to reduce the number of nodes located on a communication path by simplifying a network architecture, a plan to approximate wireless protocols to a radio channel as close as possible, and so forth.
FIG. 1 illustrates an example of a structure of an evolved UMTS mobile communication system.
As illustrated in FIG. 1, an evolved UMTS radio access network (hereinafter referred to as “E-UTRAN”) 110 has a simplified two node structure of evolved Nodes B (hereinafter referred to as “ENB”) 120, 122, 124, 126, 128 and anchor nodes 130, 132. A user equipment (hereinafter referred to as “UE” or “terminal”) 101 can access an internet protocol (hereinafter referred to as “IP”) network via the E-UTRAN 110 and a radio link.
The ENBs 120 to 128 correspond to the existing Nodes B of the UMTS system, and is connected to the UE 101 over a radio channel. The ENBs 120 to 128 discussed in the LTE system perform more complex functions than the existing Nodes B. In LTE, since all user traffics including a real-time service over an IP, such as a VoIP (Voice over IP) service, are serviced via a shared channel, an apparatus for collecting situation information of UEs and scheduling the UEs is needed, and each of the ENBs 120 to 128 functions as such an apparatus. Also, one ENB usually controls a plurality of cells, and each ENB performs adaptive modulation and coding (hereinafter referred to as “AMC”) in which a modulation scheme and a channel coding rate are determined adaptively to the channel state of a UE.
Similar to HSDPA (High Speed Downlink Packet Access) or HSUPA (High Speed Uplink packet Access, also referred to as E-DCH (Enhanced Dedicated Channel)), HARQ (Hybrid ARQ) is also performed between each ENB 120 to 128 and the UE 101 in LTE. However, since various quality of service (hereinafter referred to as “QoS”) requirements cannot be satisfied by the HARQ alone, a separate ARQ (outer-ARQ) in an upper layer may be performed between the UE 101 and each ENB 120 to 128.
Reference will now be made to a mobile communication including home cell/private network cells and public cells. A cellular communication system, an example of a mobile communication system, is widely used because it improves frequency use efficiency by dividing a service area into a plurality of cells. One cell under the control of a base station provides communication services to UEs located in the corresponding cell through the base station. In some cases, the system may be configured in such a manner that there are small-scale cells, each of which takes charge of an area smaller than the cell and is accessible by specific UEs.
An example of such small-scale cells is a cell that covers specific UEs in an area, the scale of which corresponds to any home or office, and this small-scale cell is called a home/cell/private network cell. In contrast to this, a typical cell that covers all UEs within the cell area of a base station is called a public cell. The home cell/private network cell may be used when it has an effect of reducing service costs. However, the home cell/private network cell described herein is merely an example of a cell for promoting the understanding of the present invention, and the present invention should not be construed as limited to the home cell/private network cell and the public cell. For convenience's sake, the term “home cell/private network cell” will be briefly referred to as “private cell” in the following description.
FIG. 2 illustrates an example of a typical arrangement of private cells and public cells.
Reference numeral “201” designates public/macro cells, reference numeral “203” designates private cells accessible by specific UE(s), and reference numeral “205” designates a private cell assessable by a UE 211. As illustrated in FIG. 2, the public cells may overlap the private cells. With regard to this, the public cells and the private cells may be operated with different frequency allocations or the same frequency allocation (hereinafter referred to as “FA”). A specific UE may fail in connecting a radio link to a private cell in the process of accessing the corresponding private cell. Hereinafter, a description will be given of how a UE operates when failing in connecting a radio link due to any cause in the process of accessing a private cell. Here, a failure in connecting a radio link means a state where it is impossible for a UE to communicate with a serving cell currently providing the UE with services or an ENB controlling a cell in which the UE is currently located.
FIG. 3 illustrates how a UE operates when failing in connecting a radio link to a private cell.
Reference numeral “311” designates a private cell in which the UE 321 is currently located, or which currently provides the UE 321 with services, and reference numeral “301” designates a public cell overlapping the private cell 311. The operation of the UE involves the following three steps specified by numerals “1”, “2”, and “3” in FIG. 3.
1. In the step, if the UE 321 detects a failure in connecting a radio link in the process of receiving services from the private cell 311, it operates a timer T1 that is set to a given time period.
2. In the second step, if the radio link, the connection of which has failed, is not recovered until the given time period expires, the UE 321 measures signal strengths for all cells including intra-frequency cells and inter-frequency cells. The signal strengths measured by the UE 321 may be reference signal strengths, such as pilot signal strengths. Here, the intra-frequency cells refer to cells that have the same FA as that of the cell in which the UE is currently located, and the inter-frequency cells refer to cells that have different FAs from that of the cell in which the UE is currently located.
3. In the third step, the UE 321 analyzes the measured signal strengths, and selects or reselects a suitable cell that has the best radio channel state and is currently accessible by the UE 321 (cell selection or cell reselection).
The suitable cell refers to a cell that, as a result of measurement for a radio channel state, has a measurement value equal to or higher than a minimum threshold value required for cell selection or cell reselection, called “Scriteria”, and is currently accessible by the UE. It is possible to refer to the 3GPP standard 25.304 for the Scriteria and the suitable cell, so a detailed description thereof will be omitted herein. Here, the cell refers to a public cell. In the following description, a cell does not mean a private cell, but means a public cell, unless particularly mentioned. Also, for the convenience of explanation, “selection” is used as a concept covering “reselection”.
As discussed above, if a UE operates as described in FIG. 3 when failing in connecting a radio link to a private cell, there is a problem in that the UE must measure signal strengths for all cells including intra-frequency cells and inter-frequency cells, and thus a large time delay occurs until a suitable cell is selected. Also, in the case of a mobile terminal using a battery, the power consumption of the battery is considerably increased when the mobile terminal measures signal strengths for all cells, as described in FIG. 3, which causes a problem of reducing the available time for the mobile terminal.